The present invention relates to a communications network and, more particularly, to a communications network that enables transactions based on encrypted messages between terminals.
Recently, with developments in electronic technology, there have been innovations in communications network systems such as home banking and home shopping, and office banking systems. A vital concern regarding communications network systems for handling financial transactions is guaranteeing secrecy and security for these transactions. It is necessary to increase the verifiability of a transactor's identity or a message which is transmitted and received between transactors through the communications network.
The classical types of irregularities that can occur in the transmission of transactions or messages are as follows:
(1) False reports: A sender reports not sending to the receiver although a transmission was in fact made, or the sender reports sending although no transmission was made.
(2) Forgery of documents: A receiver rewrites a communication message that has been recorded on the receiving side, or forges a communication message.
These irregularities make embezzlement possible.
In a prior art system, in order to prevent such irregularities, an enciphering program such as DES (Data Encryption Standard) is stored in each network terminal to prevent the forging of communication messages. This means that an enciphering/deciphering circuit is provided in each terminal and that a sender, using his own key, enciphers a message according to this enciphering program. The enciphered message is transmitted to a receiver terminal through a communications network. On the receiver side, the received enciphered message is deciphered in the deciphering circuit using a key word which is stored in a key memory and is peculiar to the sender and then recorded. Accordingly, assuming that the key word stored in the key memory on the receiver side has not been leaked to the outside, and that the receiver has not forged the message, there is no one other than the sender who knows the key word who can make the enciphered message. Accordingly, the verifiability of the enciphered message stored on the receiver side is very high. Therefore, a digital signature can be made on the communication document. However, when the receiver changes his terminal operation mode from the decryption mode to the encryption mode, he can make an enciphered message using the sender' s key word. Therefore, in a communication network system based on an encryption/decryption scheme, irregularities between the sender and the receiver cannot be perfectly prevented, thus failing to guarantee the security of transactions.
Copending U.S. patent application Ser. No. 739,282 filed on May 30, 1985; entitled "COMMUNICATIONS NETWORK USING AN ENCIPHERING AND DECIPHERING DEVICE" which has been abandoned as part of File Wrapper Continuation, Ser. No. 14,213, now U.S. Pat. No. 4,823,388; and assigned to the same assignee as this application discloses a communication network with LSI devices, such as IC cards, for enciphering/deciphering messages using a plurality of key words.